1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a high breakdown voltage field effect transistor equipped with an offset region.
2. Description of the Prior Art
As a prior art field effect transistor with high breakdown voltage (referred to a high breakdown voltage MOSFET hereinafter) there is known one type in which a low concentration impurity layer having the same conductivity type as that of a drain region is inserted between the drain region and a channel region.
A MOSFET of the above-mentioned type in which P.sup.+ type source and drain regions 101 and 102 are formed in an N type substrate, and a P type offset region 103 is formed between the drain region 102 and a channel region, as shown in FIG. 10, is disclosed in Supplement to the Journal of the Japan Society of Applied Physics, Vol. 44, 1985, p. 249.
In this high breakdown voltage MOSFET a high value of the breakdown voltage is attained by the widespread formation of a depletion layer in the surroundings of the offset region 103 when the transistor is in the off-state. Further, when the transistor is in the on-state, the offset region 103 acts as a drift region, and by virtue of this a current flows between the source and the drain.
In general, semiconductor devices used in the vicinity of a nuclear reactor or in the space are exposed to a high dose of radiations and develop irreparable radiation damage within the devices. When radiations such as X rays or gamma rays are incident on the gate oxide film of a MOSFET, the bonding between silicon and oxygen that was present at the interface of the gate oxide film and the substrate is broken, generating an energy level (or levels) in the interface.
These levels act as the scattering centers which obstruct the undisturbed flow of the carriers in the transistor, and deteriorates the mobility of the carriers. Due to the deterioration of the mobility the maximum drain current is reduced. When the degradation of the transistor characteristics due to exposure to radiation proceeds further, there arises the problem that the drain current diminishes and eventually the transistor operation ceases to work.